Using the NCEP/NCAR reanalysis data,the quasi-geostrophic vector Q and moist potential vorticity analyses for the heavy rainstorm in September 2004 in Sichuan and Chongqing are performed,and the distributions of quasi-geostrophic Q-vector’s divergence,vorticity and the frontogenesis function,as well as their temporal/spatial relations with the heavy rain revealed.It is found out that there are close relationship between Q-vector divergence and vertical motion.The Q-vector convergence in the low level was of benefit to the development and maintenance of vertical motion and the rainstorm’s occurrence.Conditional symmetry instability and convective instability in the mid-low level played important role in the rainstorm and therefore are probably one of important conditions to the genesis and development of the mesoscale convective system.